devel/gum: Add fish completion and port improvements

• gum binary is installed by default, so no need to use INSTALL_PROGRAM (or wrongly INSTALL_SCRIPT)
• add fish completion
• do generation of man and completions in post-build
• use correct go variable to execute cmd on post-build
• install stuff on post-install
• move man page to a valid location (${PREFIX}/man being considered as deprecated.)

PR: 272668